NMR spectrometer incorporating a re-entrant FIFO

ABSTRACT

Cyclic instrument control and data acquisition functions which are critically dependent upon synchrony are directed from a computer based system including a FIFO buffer adapted to feedback the most recently active word from its output register and re-store said word at a corresponding sequential position in the FIFO queue. To accommodate complex and interleaved control and data acquisition cycles, each FIFO word has a state portion for commanding external devices, a persistence portion for specifying the duration of selected state active in the FIFO output buffer for a desired persistence interval, and a repetition portion for specifying the number of consecutive discrete repetitions of the currently active state-persistence datum at the output of the FIFO. Termination of the cyclic sequence is accomplished at the expiration of a preselected number of complete cycles. The FIFO input address is selected automatically in accord with the number of words comprising a single complete sequence of instrument control/data acquisition functions.

CROSS REFERENCE TO RELATED PATENTS

This invention is a division of U.S. Ser. No. 107,106, filed 2-26-79,now U.S. Pat. No. 4,375,676. Copending U.S. Ser. No. 353,263 filed3-1-82, now U.S. Pat. No. 4,481,608 is also a division of U.S. Pat. No.4,375,676.

DESCRIPTION

This invention pertains to instrumentation for cyclic instrument controland data acquisition and particularly to the data acquisition andcontrol aspect of Fourier transform NMR spectroscopy apparatus.

BACKGROUND OF THE INVENTION

FT-NMR spectroscopy is acutely dependent upon modern real time dataacquisition methods. A computer based system is dictated by the multiplerequirements for control of the spin excitation process, monitoring ofsystem operating parameters, response to operator intervention,acquisition of time domain data, time averaging of such data, Fouriertransformation operations on the data and subsequent data reduction.Accordingly, considerable burden is placed upon any single processingunit to accommodate all of these functions while maintaining thenecessary data acquisition rate with required synchrony.

The most recent prior art approach to this problem segregates functionsin a multiple processor system linked by a common bus and sharing accessto a common memory. One of such processors is a special purposeacquistion processor dedicated to commanding the pulse sequence of thespectrometer, monitoring the spectrometer parameters, commanding dataconversion by the analog-to-digital converter, hereafter ADC, andperforming time averaging operations on the acquired data. The generalpurpose processor functions as a host, interpreting keyboard commandsfor general operating purposes and serving to operate upon frequencydomain data for desired data reduction, performing Fouriertransformation of the time averaged data to the frequency domain, andperforming display and output operation. The various functions areaccommodated by linked but otherwise independent processors. To assuresynchrony in the acquisition of time domain data and to satisfy thevarious demands upon it, the system is structured upon a priorityinterrupt organization. This system is especially characterized in themanner in which the acquisition processor, by its structural couplingwith the spectrometer, avoids overlap between spectrometer control anddata acquisition functions. This is accomplished in part by stacking aseries of commands in an advancing sequence buffer. (The advancingsequence buffer comprises a plurality of serially communicatingregisters for advancing a sequence of digital words toward an outputbuffer. Thus the advancing sequence buffer is known as a "first in-firstout" or FIFO buffer.) Each command word comprises an operation portionand a persistence time portion. The internal FIFO advance signal is thencontrolled by a timing means which decodes the persistence time portionand maintains the currently active command word at the output registerof the FIFO for the specified persistence time. At the termination ofthe currently active persistence interval, the FIFO content is thenadvanced. This technique permits the acquisition processor toaccommodate certain of its nonsynchronous functions during theautonomous operation of the FIFO. More important, the synchrony ofcommands to the spectrometer is carefully preserved during theoperational discharge of FIFO content. This apparatus is the subject ofU.S. Pat. No. 4,191,919, commonly assigned with the present invention.

The above-referenced apparatus preserves synchrony and frees theacquisition processor only for the period jointly defined by the numberof command words accommodated by the FIFO and by the respectivepersistence times of such words. Because the number of transientwaveforms may be large, the number of samples defining each waveformalso quite large, and the number of excitation and auxiliary commandsindefinite, the above-described prior art apparatus requires frequentservicing of the FIFO by the acquisition processor. Consequently, theinterrupt rate at the acquisition processor, although reduced (over aconventional priority interrupt organized system), may still be quitehigh and require substantial penalty in the software for accommodatingthe various levels of priority required.

Sequence buffers which operate on a first in-first out principle havebeen known in the computer art for many years for processing serial wordstrings for input-output operations. Such applications have beenaddressed to such aspects as matching disparate information processingrates of the digital processor and the communicating peripheral devicefor strings of arbitrary length.

Shift registers are another class of apparatus in which a number ofparallel bits are subject to serial advance or retard. Shift registersare also known in a form wherein the output is fed back to the input ofthe register to sequentially process the individual bits of a digitalword. These structures are employed for parallel to serialtransformation, arithmetic operation or single bit string manipulationin the prior art.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the efficiency of a systemfor concurrent data acquisition and control of instrument parameters ofa Fourier transform spectrometer, or like instrument for which the dataacquisition requirements are cyclic in nature.

In one feature of the invention, a queue of command words specifying theseries of states of the apparatus and the persistence time of suchstates is transmitted to a FIFO buffer for sequential advancement towardthe output register of the FIFO.

In another feature of the invention, each state-specifying command wordfurther includes a portion defining the number of repetitions forconsecutive executions of said command and word repetition means areprovided to accomplish the repetition of the command word for itsspecified persistence times.

In still another feature of the invention, at the conclusion of therepetition count, the command which has been most recently executed atthe FIFO output is returned to a corresponding position in the queue,whereby an endless loop of commands rotates through said FIFO outputregister.

In again another feature of the invention, the FIFO is provided withmeans for selecting the input address of the FIFO for storing a wordtherein at the physical end of the queue and transfer means are providedfor transferring the word currently active at the output of the FIFO tothe selected input position whereby a FIFO of given physical wordcapacity is adapted to accommodate a logical sequence of words fewer innumber compared to the maximum FIFO capacity.

In still yet another feature of the invention, means are provided tospecify a desired number of rotations of the queue through the FIFOoutput and termination means act in accordance therewith to conclude theacquisition of data for the currently existing measurement.

In another feature of the invention, the persistence time portion for aparticular command state and the state specifying portion for suchcommand state reside in successive FIFO words whereby the initializationof persistence time controlling apparatus may be carried out prior tothe gating to the output register of the corresponding command statespecifying portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a data acquisition system employing theinvention.

FIG. 2 is a block diagram of one embodiment of the FIFO of the presentinvention. FIG. 3a shows one choice of FIFO output word format for anNMR system of the present invention.

FIG. 3b shows another choice of FIFO output word format for an NMR datasystem of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 exemplifies the present invention applied to the data acquisitionand control functions of a modern Fourier transform NMR spectrometer 1.The latter apparatus is broadly and symbolically typified by a sample ofmatter 2 for analysis subject to a magnetic field provided by magnet 3and magnet controller 3'. One or more RF transmitters 4 and associatedmodulators and pulse formers 5 are connected to a probe assembly 6through a multiplexer 7. A receiver 8, also multiplexed to the probe 6,detects resonant signals induced in the sample 2. Wave forms of thetransient resonances are digitized by ADC 9. Instumental variables suchas sample temperature, magnetic field-frequency lock conditions and thelike are also monitored and may be altered under computer control tomaintain desired constant conditions.

A computer or system of processors 20 is also provided to automaticallyacquire and evaluate incoming information from the spectrometer 1 and aswell to issue outgoing signals to maintain desired instrumentalconditions and to control the data acquisition sequence. By way ofexample, this may require responding to temperature and field frequencylock conditions, gating on and off the transmitter and receiver,modulating transmitter signals to realize pulse sequences having desiredproperties for exciting resonance in the sample, initiating digitizationof the resonant signal at precisely defined intervals and strobing theresult of such digitization to the processor system 20.

Communication between the processor 20 and its environment comprisesinput-out (I/O) bus 22. In addition to controlling the data source(spectrometer 1) and reading data generated by the data source, theprototypical data acquisition system must communicate with mass storage,for example disc memory 24 for storage of large data arrays, operatingsystem, library and program requirements. In addition, hard copy outputmust eventually be generated as symbolized by plotter 25 or like device;visual display 26 allows monitoring of the progress of the experimentduring acquisition to correct errors, select operating conditions, or toalter the course of the experiment as appropriate after early inspectionof the data. Keyboard 23 is also connected to the I/O bus 22 to providemeans to carry out desired alterations of the course of the experiment,to schedule sequences of operations, provide required numericalparameters and the like.

A plurality of separate functions as represented by the above discussionare accommodated by requiring the computer to respond to each functionaccording to some predetermined priority hierarchy. Thus a key stroke onkeyboard 23 may generate a low priority signal to which the attention ofthe computer may be delayed without loss of data for some period becausethe human intervenor does not generate data through the keyboard at arate requiring more immediate attention. On the other hand, disc file 24may require the computer to respond within some tens of microseconds foracceptance of data from the disc file or like mass storage. Response to"data ready for input" conditions from the ADC 9 may demand a responsetime substantially shorter than the inverse sampling rate which couldreach magnitudes of order 10 MHz. Commands to initiate sampling of awave form U(t) at precise values of time, t, require very high priorityto assure availability of the computer to establish control of thevariable t within the desired tolerance (of the order of tens ofnanoseconds). All of these diverse hierarchical functions are achievedby priority interrupt system 28 responsive to interrupt signaloriginating in an external device and transmitted over the I/O bus 22.In response to any interrupt of higher priority than the task currentlyin progress, software operates to store the status of the system andinitiate the present higher priority task at the conclusion of which thenext lower priority task is resumed. Thus, a considerable number ofsteps are required at each interrupt. As thus described, thiscorresponds to a conventionally organized data acquisition system. Thepresent invention is directed to reducing the interrupt rate for acertain class of interrupts and thereby to decouple those tasks from theroutine stream of interrupt activity. Thus the burden for interruptservice and the concomitant housekeeping activities required is stronglyreduced for computer system 20.

This is accomplished in the present invention by a portion of thecontrol interface 29 as hereinafter described. The central member ofthis functional block is FIFO buffer 30 and associated control logic 32.The FIFO is a known sequence buffer which has an input register 34 andan output register 36. A sequence of digital words presented to the FIFO30 are stored in successive locations of the FIFO and FIFO control logic32 generates command pulses which advance the content of the successiveFIFO addresses from input register 34 to output register 36. Suchcontrol logic also generates status signals and controls input to thebuffer 30.

As in the previously referenced U.S. Ser. No. 907,650, the digital wordprocessed by FIFO 30 comprises a command, or state portion to select thestate of spectrometer 1 and a persistence portion which specifies theduration of the selected state. Thus, timer logic 42 receives thepersistence portion of the word from data path 46A and initiates a countdown in response thereto and at the conclusion of the persistenceinterval issues a countdown complete signal to FIFO control logic 32. Atthat point the currently active state is terminated and the content ofthe FIFO is advanced introducing the next sequential work to outputregister 36, thus initiating a new state and persistence interval at theoutput register 36. This register 36 comprises portions 36A and 36B.Reference to output register 36 is directed to the entire registeroperating as such. As in the prior art, output register 36 communicatesvia data path 46B directly or through an appropriate interface 40 withspectrometer 1.

In the present invention the datum currently resident in output register36 is transferred to input register 34 over data path 47, 47', 48 incooperation with logic sub unit 44. The use of this feature of theinvention permits complex sequences of commands to be accommodatedwithin a FIFO of modest maximum capacity. As one of its functions logicsub unit 44 includes counter means for counting the number of times afull sequence of command words rotates through output register 36. Thislogic unit 44 comprises subportions 44A and 44B for the accommodation ofcorresponding portions of the content of output register 36. Referenceto logic subunit 44 is directed to operational features of the entirelogic subunit. Logic 45 processes initialization of the rotation countfrom bus 22 and provides the necessary status signals to reflect thecondition of the system. Where the entire sequence is retained withinthe FIFO this feature permits repetition of the sequence for a presetnumber of resonance excitations, terminating the present measurement.Alternative means for terminating the FIFO operation through a haltcommand is realized by insertion of an appropriately coded control wordwhich is interpreted by timer logic 42 to halt the FIFO and set a statusbit to so indicate to the data acquisition logic.

It has been noted above that the sequence length is assumed to be lessthan the maximum FIFO capacity. It will also be noted that the maximumFIFO capacity is not necessarily a multiple of a particular selectedrepetitive command sequence. Although the invention is easily understoodon such an assumption, the general case requires a variable lengthsequence of command words to accommodate flexibility of experiments.This can be accomplished with a conventional FIFO structure but alimitation is introduced in the form of the propagation time from theinput register of the FIFO to the end-of-queue, or first availableaddress of the FIFO. In view of the above considerations, a variablelength FIFO is preferred. A representative such device is symbolicallydescribed in FIGS. 1 and 2. The FIFO length is initialized by datareceived on input 22 which is latched in a portion 50 of FIFO controllogic 32 to define a pointer to the FIFO address which is to receiveinput and to disable FIFO words not comprehended between the outputregister and the selected input address. FIFO 30 and FIFO control 32 aresubstantially conventional in organization except that the FIFO isadapted by straightforward gating means (input address pointer logic 52and input word multiplexer 52') to provide selectable length undercontrol of the processor during the initialization and cyclic operation.During ordinary (cyclic) operation the FIFO control 32 issues a strobepulse to the FIFO 30 to advance the sequence of words by transferringeach word to the next adjacent FIFO address. The strobe pulse iscommanded from a countdown timer contained within persistence logic 42.Persistence logic 42 decodes the persistence portion of the command wordas it is strobed to output register 36 and initiates a countdown. Whenthe countdown reaches zero, a signal to FIFO control 32 initiates thenext FIFO advance strobe pulse. Persistence logic 42 also detects alegal halt (for example, a maximum or a minimum persistence time code)and sets a FIFO halt flag to so inform the processor.

The implementation of a sequence of control words which exceeds themaximum capacity of the FIFO requires partial operation in anon-feedback mode more fully described in above-referenced U.S. Pat. No.4,191,919. Thus, operation in this partial non-feedback mode does notresult in transfer of the content of output register 36 to the FIFOinput to maintain the sequence. In this mode, imminent FIFO underflow isindicated by a status bit which initiates an interrupt to the processor20 for reloading the FIFO 30 with the next required word of the commandword sequence. In this mode of operation the control word repetitionfeature of the present invention is operative to reduce the interruptrate by maximizing in the FIFO 30 the number of discrete command stepsto be realized by the spectrometer 1. A simple example is apparent inthe use of the repetition feature of the present invention to completelyspecify in a single word the entire set of commands to an ADC toinitiate data conversion and the precise repetition frequency thereof.(In digitization of a complex waveform for Fourier decomposition, thewaveform must be sampled a large number of times at precisely spacedintervals.) The present invention significantly improves the capabilityof the data acquisition/control structure of above-referenced U.S. Pat.No. 4,191,919 in that non-redundant and cyclic operations may beconveniently joined. For example, it may be desired to repetitivelyexecute a subset of states within a given measurement. The simplerepetition of the data conversion operation described above may includein some instances a more complex subset of instructions including dataconversion of a single point, such subset to be repeated for the largenumber of discrete data points necessary to characterize the desiredtime domain data.

Turning now to FIG. 3a, there is shown one representative choice offormat for a 32 bit FIFO output word ultimately directed to outputregister 36. A persistence interval is specified with a timing countsubfield comprising 10 bits and a time base subfield of 4 bits. The timebase subfield supplies a power of ten which multiplies the timing countsubfield. (The particular word division and bit allocation isillustrative only.) The remaining group of 18 bits are divided betweenspecifying both the state of the apparatus and the number ofrepetitions. The allocation of bits for state specification is dependentupon the possible number of distinctive substates which are concurrentlycompatible. Ordinarily, an FT-NMR system is susceptible to a very fewconcurrently specified substates which this basis, easily divided intotwo independent command groups along the lines of Table 1 (the commandsof Table 1 are illustrative only and a detailed explanaton thereof isnot essential to understanding the present invention. A 5 bit subfieldis sufficient to specify any one of the desired operands. FIG. 3aillustrates such a choice wherein one bit of a 5 bit operand sub-fieldis available to designate either of two such command groups. Theremaining 4 bits can specify up to 16 combinations of the 8 substatesavailable in either group. Thus, a limited degree ofmicroprogrammability is available while a substantial area fornon-compatible substate superpositions (errors) is eliminated. In theprior art a 16-bit subfield was allocated to the specification of outputcommands. In such case the functions were allocated on a dedicatedbit-per state basis permitting the concurrent specification of up to 16substates from the 16-bit field where in fact nearly all of the 2¹⁶ -1possible composite states would be meaningless. For a general apparatussuch a large field is, of course, appropriate, although it is notoptimum where a substantial portion of mathematically possiblecombinations, here 2¹⁶ -1, are not physically compatible.

    ______________________________________                                        Command Group    Command Group                                                Bit = 0          Bit = 1                                                      ______________________________________                                        OBSERV XMTR ON   OBSERVE RCVR                                                 DECOUPLER ON     ADC CONVERSION START                                         RF 90°    ADC MODE 1                                                   RF 180°   ADC MODE 2                                                   MODULATION MODE A                                                                              OBSERVE/LOCK                                                 MODULATION MODE B                                                                              QUADRATURE/                                                                   NO QUADRATURE                                                HOMOSPOILING     DECOUPLER 90°                                         DECOUPLER HIGH/LOW                                                                             DECOUPLER 180°                                        ______________________________________                                    

An intermediate choice is shown in FIG. 3(b) where a 10-bit operandfield is shown. In such case, and assuming a multiple command groupdivision similar to Table i, there is available a bit-for-functionallocation for each of the 8 substates, a group designation bit and anextra bit which could be used to expand the number of groups or thenumber of functions. In this format a selected operand may well beincluded in more than one group of operands.

These format choices for the command, or state portion of the worddetermine the field available for the repetition count field. For theshort command field of FIG. 3(e), a larger repetition field isavailable. Thus, a 13-bit repetition field permits specifying that thecurrent state be repeated up to 2¹³ -1 times in addition to the initialstatement of the operand. A repetition of zero implies that thecurrently stated operand is not repeated at the end of the correspondingpersistence interval, and the FIFO content is than advanced to deliverthe next sequential word to the output register. These long repetitioncount situations are useful for specifying a corresponding number ofsuccessive identical state commands, e.g., conversion commands to theADC for sampling a time-dependent wave form at precisely delineatedintervals, as specified by the persistence interval. It should be notedthat for ADC command purposes a pulse is ordinarily delivered to commandthe ADC as, 5 for example, to reset the ADC, to initiate the conversionand the like, while the persistence interval in that circumstance may beinterpreted by the apparatus as an interval between pulses.

For the longer command field of FIG. 3(b) the repetition count field isabbreviated in length requiring a logarithmic-linear interpretation. Arepetition mode select bit designates whether the 6-bit repetitionfield, n=0, 1 . . . 2^(n) -1 specifies 0 to 63 repetitions, or on theother hand, the selection bit may designate that n is interpreted as2^(k-m) for long repetition counts, where k is some convenient maximumexponent and m is the desired number of bits (k>m).

The several portions or subfields described above do not necessarilyreside in the same word. For example, the timing subfield may reside inthe next adjacent word with respect to the output register.

Another embodiment of the FIFO based systems described above lacks apersistence time decoding structure but realizes the persistenceselection result from a repetition function. The repetition field is ineffect the time counter and the time base is supplied by the processorclock available over bus 22, or preferably a faster internal FIFO clock.The treatment of the control word as either discrete repetitions or as astate continuously persisting for an interval of n time units isselected in accord with a prescribed bit of the command word andimplemented within the interface 40.

In one embodiment initialization may take the form of initializing oneof dual countdown registers, the remaining one being currently active.At the conclusion of the currently active countdown, the countdown=0 andrepetitiion bit in the active state will gate the second countdown onthe first countdown off. Where no repetition bit is encountered, thesequence is advanced in straightforward fashion at the termination ofthe currently active state.

Another embodiment for achieving persistance selection from repetitionis realized in a more general type of sequence buffer whereinbidirectional internal transfer means are provided for either advancingthe FIFO content toward the output register 36 or alternatively,teransferring the content in the opposite direction by at least oneaddress increment prior to advancing the sequence again toward theoutput register. In this form, repetition is realized in conjunctionwith a latched output register 36 wherein output register 36 is loaded,the repeat command is decoded (as by a logic unit analogous to unit 42)and the reverse transfer accomplished while the output buffer content ismaintained. In this realization, the repetition count field initializesa countdown circuit which maintains the prescribed state in outputregister 36A until the countdown is complete, At that point, the nextstrobe pulse advanaces the sequence of control words transferring thenext output word to the output register. Because the word nowtransferred to output register 36 is the same word previously shiftedinto output register 36 then returned to the last FIFO address, acountdown flag set by the countdown circuit causes a double transferforward. Thus, the repeated command is transferred to the feedback pathto input buffer 34 for maintaining the sequence.

Although an NMR spectrometer has been selected as the illustrativeexample for the present invention it will be obvious that any dataacquisition system which must issue periodic commands and acceptaperiodic interrupts may use the present invention profitably. It willbe apparent that many changes could be made in the above method andapparatus and many apparently different embodiments of this inventioncould be made without departing from the scope thereof; it is thereforeintended that all matter contained in the above description and shown inaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

I claim:
 1. In an NMR spectrometer utilizing a stored program digitalprocessor for execution of a desired sequence of operations, a firstclass of said operations directed to controlling the status of saidspectrometer and another class of said operations directed to acquiringdata from said spectrometer,a digital sequence buffer apparatusincluding an input register and an output register and a plurality ofintermediate registers for retaining a plurality of digital words, eachsaid word comprising an operand portion for specifying the instantaneousconfiguration of said spectrometer and a persistence portion forspecifying the persistence interval of each operand, and a repetitionportion for specifying the desired number of sequential repetitions ofthe currently active word, first means responsive to the persistenceportion of said digital word for sustaining the currently active operandfor the desired persistence interval, second means responsive to therepetition portion for re-initiating the currently active operand at thecompletion of the persistence interval thereof, said second meansoperative to continue said repetitions until the operand has beenrepeated for the desired number of repetitions, third means foradvancing the content of said buffer toward said output register of saidbuffer apparatus when the persistence portion of the currently activeword has been completed for the number of sequential repetitionsspecified by said repetition portion, and means for re-introducing saidsequence of instruction words to the input portion of said sequencebuffer whereby said sequence is preserved.
 2. In an NMR spectrometerutilizing a stored program digital processor for execution of a desiredsequence of operations, a first class of said operations directed tocontrolling the status of said spectrometer and another class of saidoperations directed to acquiring data from said spectrometer,a digitalsequence buffer apparatus including an input register and an outputregister and a plurality of intermediate registers for retaining aplurality of digital words, each said word comprising an operand portionfor specifying the instantaneous configuration of said spectrometer anda persistence portion for specifying the persistence interval of eachoperand, first means responsive to the persistence portion of saiddigital word for sustaining the currently active operand for the desiredpersistence interval, second means responsive to the repetition portionfor re-initiating the currently active operand at the completion of thepersistence interval thereof, said second means operative to continuesaid repetitions until the operand has been repeated for the desirednumber of repetitions, third means for advancing the content of saidbuffer toward said output register of said buffer apparatus when thepersistence portion of the currently active word has been completed, andmeans for re-introducing said sequence of instruction words to the inputportion of said sequence buffer whereby said sequence is preserved. 3.The NMR spectrometer of claim 2 comprising cycle counter means forcounting the number of times said complete sequence has beenre-introduced to the input portion of said sequence buffer.
 4. The NMRspectrometer of claim 3 comprising means responsive to said countermeans for causing said FIFO to halt after a pre-determined number ofre-introductions of said complete sequence.
 5. The method of operatingan NMR apparatus to control same and to acquire data therefrom,comprisingdefining a complete sequence of descriptors of operationalstates of said apparatus required to cause excitation and detection ofgyromagnetic resonance conditions according to a pre-selectedoperational procedure, prescribing for each state of said apparatus, acorresponding persistance interval for each state to be maintained,establishing each state and maintaining said state for its correspondingpersistence interval, re-introducing said descriptor of each said stateand the corresponding persistence parameter in said sequence wherebysaid complete sequence is preserved for repetition of execution of saidcomplete sequence, repeating said step of establishing, maintaining andre-introducing.
 6. The method of operating an NMR spectrometer accordingto claim 5 wherein said step of prescribing comprises prescribing foreach state a repetition count for causing said state persisting for saidcorresponding persistence interval to repeat the number of repetitionsspecified by said repetition count.
 7. The method of claim 5 or 6comprising counting the number of times said complete sequence isre-introduced for repetition.
 8. The method of claim 7 comprisingterminating said step of repeating when said number of times meets apre-selected criteria.